$(window).ready(function() {
    var winHeight = $(window).height();
    var clockHeight = $('.clock-container').height();
    var clockMargin = (winHeight - clockHeight) / 2;
    $('.clock-container').css('padding-top', clockMargin);
});

$(window).resize(function() {
    var winHeight = $(window).height();
    var clockHeight = $('.clock-container').height();
    var clockMargin = (winHeight - clockHeight) / 2;
    $('.clock-container').css('padding-top', clockMargin);
});